The mechanism of the glutamate inhibition of glutamine synthetase induction in chick embryo retina is being investigated. The latter effect appears to be age-dependent; L- and D-isomers of glutamate appear to act somewhat differently, the latter having an effect on both induction and pre-induced enzyme, whereas the L isomer only affects the inductive process. In addition, a variety of other glutamate analogues as well as Krebs cycle intermediates are also being investigated. The mechanism of the glutamate induced morphologic damage to chick embryo retina is also being investigated both by light and electron microscopy. Also being investigated are the possible physiological functions of the glutamyltransferase activity of glutamine synthetase in the developing chick embryo retina. We are also studying various biochemical changes which may account for apparent reduced inducibility in disaggregated retinal tissue compared to whole retina. Attempts are being made to pinpoint which phase of the inductive process, if any, is affected by tissue disaggregation. Attempts are also being made to grow out subpopulations of retinal cells in an effort to determine whether glutamyltransferase inducibility is localized in one or more subpopulations of these cells.